[Bug rtl-optimization/15242] pessimization of "goto *"

[zlomj9am at artax dot karlin dot mff dot cuni dot cz]

------- Additional Comments From zlomj9am at artax dot karlin dot mff dot cuni dot cz  2004-07-16 08:42 -------
Subject: Re:  pessimization of "goto *"

> > Adding an new flag is possible. However, I do not see why you explicitelly want
> > to disable reorder-blocks.
> 
> We need it for the dynamic superinstruction optimization (aka
> selective inlining, PLDI'98 p. 291) in Gforth and further
> optimizations building on that (other people working on efficient
> interpreters/dynamic code generators have the same problem; e.g., I
> recently talked to Etienne Gagnon (http://sablevm.org/) about this;
> qemu also uses -fno-reorder-blocks).
> 
> For dynamic superinstructions we need to ensure that the machine code
> corresponding to source code between two labels is between the
> addresses that gcc produces for the labels.  The flag
> -fno-reorder-blocks almost gives us that (but the sharing of the
> "goto *"s breaks that).

I see.

> Even for just plain interpreters (without dynamic superinstructions),
> the sharing of the "goto *"s resulting from the merging of code after
> conditional jumps (as shown in the simplified testcase) will often
> reduce performance significantly through lower BTB accuracy.  So for
> performance one probably wants to use -fno-reorder-blocks there,
> unless this results in merging the "goto *"s, as it does now.

I see. So duplication of computed jumps probably should be at -O
and have a special flag too.

Josef


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15242